Reversible phase modulating element

ABSTRACT

A pattern generating device includes a plurality of biaxial birefringent irregular ferroelectric crystal elements, each crystal being cut such that mutually opposing planes are normal to any one of the a-, b- and c-axis that a thickness between the planes is that of a half-wave plate, said crystal elements being arranged in the form of a matrix on an identical plane normal to incident light, a required information pattern being recorded on a photosensitive medium in a manner that elements corresponding to the information pattern are manipulated by a threshold voltage applied to the Z-planes of the respective elements so that a large-capacity information recording method can be carried out.

United Statc Fukuhara 1 Dec. 25, 1973 [5 REVERSIBLE PHASE MODULATING3,069,973 12 1962 Ames 350 150 ELEMENT 3,559,185 1 1971 Burns et al350/150 Inventor:

Assignee:

Filed:

Appl. No.:

Foreign Application Priority Data Dec. 9, 1970 Japan 45/108530 Dec. 25,1970 Japan 45/130687 US. Cl 350/150, 340/173.2, 350/35,

Int. Cl. G02f 1/26 Field 61 Search 350/147, 149, 150,

References Cited UNITED STATES PATENTS 5/1960 Anderson 350/150 X 8/1971Cummins 350/150 6/1971 Kumada et al 350/150 Primary Examiner-John K.Corbin Attorney-Craig, Antonelli & Hill ABSTRACT A pattern generatingdevice includes a plurality of biaxial birefringent irregularferroelectric crystal elements, each crystal being cut such thatmutually opposing planes are nonnal to any of the a-, band c-axis that athickness between the planes is that of a halfwave plate, said crystalelements being arranged in the form of a matrix on an identical planenormal to incident light, a required information pattern being recordedon a photosensitive medium in a manner that elements corresponding tothe information pattern are manipulated by a threshold voltage appliedto the Z- planes of the respective elements so that a largecapacityinformation recording method can be carried out.

9 Claims, 14 Drawing Figures Pmmmnecas I975 3.781.084 SHEET 10$ 7 FIG. I

DIFFRACTION LIGHT INTENSITY CENTER x I FIG. 40 FIG 14b PATENTEDUEBZSIHB3.781.084 I SHEETHIF'! v Bro F REVERSIBLE PHASE MODULATING ELEMENTBACKGROUND OF THE INVENTION The present invention'relates to a patterngenerating device utilizing a special ferroelectric crystal, and amethod of recording the generated pattern.

U.S. Pat. No. 3,559,185 discloses that a device comprising a combinationof quarter-wave Z plate a Gd, M00.);, is abbreviated GMO, plate in whichthe thickness between the mutually opposing Z-planes corresponds to thethickness'of a quarter-wave plate and a quarter-wave plate, each beingmade of the single crystal of Gd, M00.,),, arranged between a polarizerand an analyzer to be used as a light shutter device, and the lightshutter device is arranged in the form of a matrix in a two-dimensionalspace normal to incident light, thereby causing the respective elementlight shutter devices to generate predetermined patterns by lightshutter action.

The device of the above construction, however, has been disadvantageousas stated below.

I. In practical use, the light incident on the quarterwave Gd, M plateis not normally incident but is incident at a slight inclination. Inaddition, in order to expose different positions on a hologram medium tolight, light should be irradiated upon the element light shutter devicein a deflected manner and, hence, it is not always normally incident. Incase of inclined light incidence, the optical path length of thepermeating light varies due to the biaxial birefringent property of theemployed ferroelectric substance Gd,( M00 so that the prior art devicedoes not perfectly function as quarter-wave plate. Differences areaccordingly produced in the ratio of intensities for switch-on and -offstates of the particular light shutter. More specifically, from theviewpoint of crystal optics, Gd, M00 is a biazial and birefringentcrystal. Refractive indexes the crystal axes d, b and mcdi rec tions,respectively,

are

u n 4 X10 with respect to light of a wavelength 6,328A. I-le- Ne laserAs calculated from them, in order to secure a ratio of, intensities forthe switch-on and -off states of at least I 10'' as is necessary forpractical use, the deviation in the direction of irradiation upon thequarterwave Gd, M00 plate should be made within 120 The degrees of anglescarcely differ for light in the visible region (4000A. 7,500A.), anddouble the value is never exceeded. Therefore, in order to strictlyoperate the light shutter device and the pattern generating device, theincident direction of the irradiated light upon the quarter-wave Gd, M00plate should be restrained within at most I 20' with respect to thenormal direction of the particular quarter wave Gd, M00 plate.

' 2. Further, for a pattern generating device constructed by, asdescribed above, arranging the light shutter devices within an identicalplane normal to the irradiated light in the form of a matrix, when it isintended to record the Fourier transformation image of a generatedpattern at a focal position by means of a Fourier transform lens havinga focal length -f, there is a direction in which diffracted light beamsamong light beams permeating a light shutter device corresponding to thegenerated pattern image intensify one another. I

For a better understanding of such a device and the principles of thepresent invention attention is directed to the drawings, wherein thecharacteristics of such patterns and the embodiments of the presentinvention are shown.

BRIEF DESCRIPTION OF THE DRAWING Y-cut half-wave Gd, M00 plates of thepresent invention, respectively;

FIG. 3 is a counter map showing intensity distributions in thereconstructed light of a double-exposure hologram in the case of using aprior-art quarter-wave plate and before and after the inversion ofpolarization, the distributions of which depend on incident angles ofirradiated light;

FIGS. 4a and 4b are model diagrams showing lattice states dependent onthe senses of spontaneous polarization of Gd,( M00 unit lattices,respectively;

FIG. 5 is a diagram of the refractive-index curved surface of biaxialbirefrigence of an irregular ferroelectric substance;

FIG. 6a is a perspective view showing a method of using a Z-cuthalf-wave ferroelectric plate according to the present invention;

FIG. 6b is a perspective view showing a method of using a Y-cuthalf-wave ferroelectric plate according to the present invention;

FIG. 7a is a diagram showing a hologram recording apparatus which uses apattern generating device embodying the present invention;

FIG. 7b is a diagram for explaining a method of reproducing a hologramwhich has been prepared by the hologram recording apparatus in FIG. 7a;

FIG. 8 is a diagram showing a method of forming a hologram with apattern generating device of another embodiment of the presentinvention;

FIG. 9 is a diagram showing a method of preparing a hologram with apattern generating device of still another embodiment of the presentinvention; and

FIG. 10 is a diagram showing one aspect of manipulating the patterngenerating device of the present invention.

Referring to FIG. 1, an intensity distribution appears which has themaximum peak at the center and some peaks gradually decreasing withdistances from the 'center in FIG. 1 character 1 represents theintensity of diffracted light,'d the distance between the centers of thelight shutter devices, and A the wavelength of incident light In case ofthe intensity distribution having such peaks, the photosensitivity of animage recording medium is such that the medium is saturated forexcessively intense light, whereas it is insensitive to weak light. Withthe system of the above pattern generating device, it is, accordingly,difficult to faithfully record and reproduce the generated pattern. Itis, therefore,

desirable that variations in the spatial intensity distribution of theimage to be recorded are made as small as possible.

SUMMARY OF THE INVENTION Another object of the present invention is toprovide a large-capacity recording method with the abovementionedpattern generating device.

The present invention uses a single crystal belonging to a special groupamong ferroelectric substances, such as Gd M and potassium dihydrogenphosphate hereinbelow abbreviated as KDP the ferroelectric substancebeing cut so that the mutually opposing planes of the crystal may benormal to any one of the a-, band c-axis, respectively, and that thethickness between the opposing planes may be one at which refractiveindexes n and n for light of a wavelength A permeating through theparticular crystal and having polarization planes parallel to the aandb-axes produce a difference of half-wavelength from each other.

According to the present invention, a half-wave plate of theferroelectric substance e.g., Gd, M009 cut as described above isarranged in such a manner that the cut plane thereof has the a-axis orb-axis within the plane and made parallel to the polarization plane ofan incident linear polarization. The phase change of half-wavelength ofpermeating light is manipulated by a voltage which is applied in thedirection c-axis of the half-wave ferroelectric plate. Accordingly, ifthe phase change is recorded and detected by any method, an electricsignal applied to the half-wave ferroelectric plate may be convertedinto a binary light signal. For example, in a device wherein a pluralityof half-wave ferroelectric plates are arranged within an identical planenormal to coherent irradiated light in the form of a matrix, it ispossible to record information as a hologram in such a way that therespective element half- I wave ferroelectric plates are manipulated soas togenerate a predetermined pattern by applying the voltages, lighthaving permeated through the plates is made an object beam, and that aseparate reference beam is irradiated onto a' photosensitive medium in amanner superposed upon the object beam Embodiments .1 and 2 In thiscase, a reproduced image appears as a pattern of the phase changes. Inaddition, when an interference pattern with a flux of light emitted froman identical source of light is produced, it is also possible to recordand detect information in the form of changes of brightness Embodiment 3in case a hologram is prepared by means of the pattern generatingdevices with the half-wave ferroelectric plates of the present inventionin a manner that light including a predetermined signal is informationlight,

the recording of an on" condition is effected by double exposure of thehalf-wave ferroelectric plates with their polarity the sense ofspontaneous polarization left as it is, while the recording of an ofP'condition is effected by double exposure with the polarity inverted thesense of spontaneous polarization being changed by 180 However, when thehologram is prepared with the crystal half-wave ferroelectric platepermeating light made the information light and reproduction is donetherefrom, information light beams opposite in phase to each other aredoubly and simultaneously reproduced from parts corresponding to the offstate, and hence, the intensity is a cancelled low one. When lightintensities 1,, reproduced or reconstructed from the double-exposurehologram with the polarity inverted and those 1 without the inversionare evaluated, results in FIGS. 20, 2b and 3 are'obtained. The

figures are given in the form of ctour contour of the reconstructedlight intensities 1,. and L, The dependency of the intensities isrepresented only in the range of 0 5. dz 90 because of the followingsymmetry expressed interms of the angle 4) defined between a planecontaining an incident light ray as well as the normal of the crystalsurface and the a'-axis or b-axis Although natural, it is desirable thatan angular region of a large value is wide for I,,,,, whereas that of asmall valuefor 1 From the viewpoint of practical use, the

' plate, a Y-cut plate a single-crystal plate in which two opposingparallel planes are cut normally to the a-axis or b-axis and a prior artquarter-waveplate, respectively. In the figures, 0 represents an angledefined between the normal of the cut plane and the incident light,while designates, as referred to above, the angle which the planecontaining the incident'light ray and the normal of the cut planedefines with the a-axis or b-axis a In case of the Z-cut plate, asapparent from FIG. 2a, the direction of d) 45 has the narrowestallowable range of 6 of approximately 2.5 1n the directions of 0 and180, the allowable ranges are wide.

As apparent from FIG. 2b, the Y-cut plate is much more convenient thanthe Z-cut plate. The allowable range of 0 is the narrowest at (b 0, andit may be taken up to 0 12.

In case of the prior art quarter-wave plate, as apparent from HO. 3,although the direction of 45 is the widest in the allowable range, 0does not reach 2 Crystals employed in the present invention as statedabove, are Gd M000 KDP Rochelle salt, ammonium cadium sulphate,methyl-aluminum sulphate dodecahydrate, and crystallographicallymonomorphic substances of Gd M00 which have the formula of R, R O 3Mo, WO where R and R each. represents one rare earth element, x a value of 0to 1.0, and e a value of 0 to 0.2. The ferroelectric substances vhavesuch a property that, upon applying thereto an electric field or astress which exceeds a threshold value inherent to the substance (theelectric field of the fixed value shall be termed a coercive electricfield, while the stress of the fixed value, a coercive stress the senseof the electric polarization of the particular crystal is inverted orchanged by 180 Moreover, simultaneously with the 180 inversion of' thepolarization, a lattice deformation is generated which, as shown inFIGS. 4a and 4b, is equivalent to the replacement between the aandb-axes. Herein, ferroelectric substances generating no deformation inthe crystal lattice in dependence upon the positive and negative sensesof the polarization shall be termed regular ferroelectric substances,while those generating the de' formation shall be termed irregularferroelectric substances. The above-mentioned Gd; M000 and KDP belong tothe irregular ferroelectric substances, while c represent crystal axes,while n n DESCRIPTION OF THE PREFERRED EMBODIMENTS Description will nowbe made of the embodiments of the present invention:

EMBODIMENT l Illustrated in FIGS. 7a and 7b is an embodiment in which apattern generating device of the present invention is utilized for ahologram forming apparatus.

A plurality of Gd,( M00 crystal plates 61 in each of which, as shown inFIG. 6a, the thickness between the Z-planes planes normal to the c-axiscorresponds to the thickness of a half-wave plate and transparentelectrodes 62 are arranged on both the Z-planes, are arranged in theform of a matrix such that the Z planes of the respective elements arelocated on an identical plane nearly normal to incident light, and thatthe polarization plane of incident linear polarization and the aorb-axis of the Gd, M000 crystal plate are parallel to each other. Thus,the pattern generating device FIG. 7a, the pattern generating device 700is disposed between two lenses 73 and 74. Utilizing coherent lightradiated from a laser light source 75, a hologram of a pattern generatedby the generator device 700 is recorded by object light and referencelight onto a hologram recording medium 76 which is arrangedsubstantially at the focal position of the lens 74. In FIG. 7a,

numerals 77 and 78 represent semi-transparent mirrors, while 79 is alens of a focal distance f, as is arranged at a position of f +1}, infront of the lens 73 of a focal distance f,). In the above hologrampreparing apparatus. the pattern generation is carried out by applying athreshold voltage a voltage required to invert spontaneous polarizationby 180 of the respective Gd M00 crystal plates through the transparentelectrodes on the Z-planes of said Gd; M0O,) crystal plates. Thethreshold voltage may be in any form insofar as the voltage component inthe direction of the caxis is equal to said threshold voltage. Forexample, even if the voltage is applied through the electrodes disposedon the Z-planes as in the above, the pattern generation may also beaccomplished by irradiating an electron beam which establishes anelectric field equal to the threshold voltage threshold electric fieldin 1 crystal plate with its polarization plane parallel to the z-axis orb-axis of the Gd M00 cyrstal plate, an effect equivalent to thereplacement between the aand b-axis may be imparted to the Gd,( M000crystal plate by applying the threshold voltage to the Z-planes. Thus,the phase of a permeating linear-polarization 700 FIG. 7a) isconstructed. Further, as shown in 6 light beam having a polarizationplane parallel to the aor baxis is modulated. Therefore, the generatedpattern recorded on the hologram medium 74 by means of the patterngenerating apparatus comprising the Gd, M000, crystal plates of suchconstruction and constructed as in FIG. 7a, is obtained as an image atthe position in the pattem generating apparatus at the recording of thehologram in such a way that, as illustrated in FIG. 7b, coherent lightis irradiated upon the hologram recording medium 76 in a directionopposite to that of the reference light beam for the preparation of thehologram.

When, as shown in FIG. 6b, an interferable-light beam generating source1' having a Brewster angle window is employed, it is not necessary toarrange the polarization plate in front of or at the back of themutually opposing end faces of the Gd, M00 crystal plate as isillustrated in FIG. 6a. The Gd,( M00 crystal plate shown in FIG. 6b iscut such that the opposing end faces are normal to the b-axis or thea-axis and that the thickness between the end faces is equal to that ofthe half-wave plate. Provided on the mutually opposing Z-planes areelectrodes 62, through which a voltage corresponding to the 180inversion of the spontaneous polarization of the Gd M00 crystal plate isapplied.

EMBODIMENT 2 There will be described an embodiment which is used as anoriginal plate for recording a hologram. A pluralityof Gd M00 cyrstalplates are arranged in the form of a matrix such that the light incomingand outgoing planes thereof are located on an identical plane nearlynormal to incident light, each element being cut so that the thickness dbetween both the front and back end surfaces normal to the c-axis aredefined by:

with respect to the wavelength used, and being provided on therespective Z-planes with transparent electrodes. As illustrated in FIG.8, light from an interferable-light source is divided into two parts.One of the parts is transmitted through a lens 89 at the back of asemi-transparent mirror 87 arranged at an angle of 45 Further at theback of a pattern generating device 800 consisting of the Gd M00 crystalplates a reflector 810 is arranged. The irradiation light beam is passedthrough the pattern generating device twice via a semitransparentmirr'or 811, a lens 83, the pattern generating device 800 constituted bythe Gd, M004); crystal plates and the reflector 810. Thereafter, thebeam is irradiated upon a hologram medium 86 by the semitransparentmirror 811, to focus an interferen'ce image with the reference lightfrom the interferable-light source 85. a

Both the foregoing embodiments l and 2 adopt the Fourier transformhologram recording system in order to enhance the recording density ofthe holograms. With this system, in case the bit arrangement i.e., the

v arrangement of the Gd M00 crystal plates on the pattern generatingdevice) is at equal spacing and in the form of a matrix, theconcentration of the intensity distribution in the diffracted light ofthe permeating light beam from the respective bits occurs on thehologram recording medium.

This is an objectionable phenomenon in the characteristic of thephotosensitive material, and in that the local presence of informationshould be avoided. To avoid the concentration, there is a method inwhich the phase on the information plate is disturbed independent of theon" and off signals. The pattern generator of the present invention maysimultaneously have the function of the phase impartation. Morespecifically, first, the voltage of random distribution is applied andthe exposure in once carried out. Next, the polarity is inverted only atthe bits intended for the off state,

and the second exposure is carried out. It is necessary that the randomphase distribution to be first imparted be a previously made oneassociated with information patterns, so as not to become a regulararrangement (for example, being entirely in-phase, being in-phase atevery second line, etc.) in either of the two exposures. It is anadvantage that the intensity concentration may be thus avoided. Y

EMBODIMENT 3 As embodiment will now be explained in which input electricsignals are detected and recorded in the form of a brightness pattern.

As shown in FIG. 9, a convex lines 109, a semitransparent mirror 11] anda reflector 110 are arranged into a Twyman interferometer. One of lightbeams from an interferable-light source 105 is caused to impinge uponthe interferometer through the convex lines 109. A pattern generatingdevice 101 made of Gd M000 crystal plates, each being cut normally toany crystal axis and each thickness between both the front and back endsurfaces corresponding to that of a quarterwave plate, is interposedbetween the semi-transparent mirror 11] and the reflector 110 as in thefigure; An image focusing lens 104 is so arranged as to focus a patterngenerating image of the pattern generating device I01 onto a screen 106.If the optical path is previously adjusted so that the surface of thescreen may become bright when a voltage applied to predetermined Gd bitsof the pattern generating devices are of an arrange ment at regularintervals on said generating devices. This induces a variety ofobjections in, for example, maldistribution of the generatedinformation, the balance of the intensity with the reference light, andlimitation on the appropriate exposure region of sensitive materials.

It is known that, when the phases of light passing through therespective Gd M009 crystal plates of the pattern generating device arerandomly disturbed, the intensity concentration on the hologramrecording medium is weakened. Such an idea is stated by C.B. Burckhardtin a paper entitled Use of A Random Phase Mask for the Recording ofFourier Transform Holograms of Data Masks" in Applied Optics" publishedMarch, 1970, Volume 9, Pages 695 to 700. The method for randomdisturbance is realized in case of the embodiments l and 2 by arranginga random phase shifter. which makes the phases random. at the back ofthe pattern generating device. When, in the embodiments l and 2, thephase distribution given by the random phase shifter is fixed and thegenerated patterns are changed in succession, the information lightintensities on the hologram recording medium are changed. Although theintensity concentration is weakened on the average in comparison with acase without using the fixed random phase shifter, objectionablepatterns are present in the respective times of generated patterns. Itis too troublesome that, in order to overcome such disadvantage, thefixed random phase shifter is replaced at each generated pattern. In thefollowing embodiment, description will be made of a method of recordinglarge-capacity information in the embodiments l and 2, which method isimproved in the above respect.

EMBODIMENT 4 The aspect of performance of this embodiment is illustratedin FIG. 7a, while a partial detailed view thereof is given in FIG. 10.

In FIG. 7a, a light beam emanating from a laser light source 75 isdivided into two parts by a beam splitter 77. One of the divided lightbeams is made a thicker parallel light beam as information light bymeans of a beam expander 73. It passes through an information patterngenerating device 71 having also the function of rendering the phases ofthe respective bits random said device 71 being hereinbelow termed therandom phase shifter Thereafter, it is condensed by a Fourier transformlens 74 to a hologram forming-plate 76 which is arranged on the focalplane of the lens. The other light beam separated by the beam splitteris used as a reference beam, and is reflected by a reflector 78.Thereafter, it impinges on the hologram recording medium 76 with anangle defined thereto, and interferes with the information light to forma hologram pattern including a predetermined information.

The random phase shifter 700 of embodiment 4 serves also as theinformation pattern generating device, and it is constructed as apattern generating device lll shown in FIG. 10. More specifically, eachGd M00 crystal plate 111 is formed such that both the front and backprincipal planes are orthogonal to the c-axis that the thickness betweenboth the principal planes is an odd multiple of the thickness of ahalf-wave plate for the wavelength of the ligh used for example 0.3;1.for the He-Nelaser light 0.6328 and that it has an area of 250 X 250 1.On the mutually opposing principal planes Z-planes transparentelectrodes are arranged which are provided by, e.g., evaporating SnCl, Nsuch Gd M009 elements the number being increased or decreased dependentupon the state of use they have the relation of rows and columns. Thus,the

pattern generating device 111' is made. Lead wires 1111 connected to theabove-mentioned transparent electrodes of the respective Gd M00 crystalplates are connected to an electronic computer 1112 which stores thereinN numerals of l or 0'? arranged in a random order. Bits Gd; M009 crystalplates corresponding to an information pattern to'be generated in theinformation pattern generator and random phase shifter apparatus, areturned on" through the electronic computer. Subsequently, a voltage of300 to 400 V is applied through the electronic computer to the Gd, M00crystalplates which correspond to bits to be turned off. An image of therandom phase is thereby .formed on the hologram sensitive plate 76.Subsequently, a voltage opposite in polarity to the above appliedvoltage is applied to the Gd, M000 crystal plates which correspond tothe bits to be turned off, thereby inverting their polarization. Animage is again formed on the hologram sensitive plate through'theapparatus. Then, a hologram image of the predetermined informationpattern is formed on the photosensitive plate.

While the principal planes of the crystal plate of the above Gd M000crystal plate utilize the Z-planes, the Y-planes planes cut normally tothe aor baxis of the crystal may also be utilized. in this case, thevoltages should, of course, be applied through the mutually opposingZ-planes as in the foregoing.

Further, since the laser light source employed in the present embodimentutilizes laser light 0.6328u from a He-Ne gas discharge tube having aBrewster window, the emitted light is of P-polarization. It is,therefore, unnecessary to arrange a polarizer in front of theinformation pattern generating device. In general, however, in casewhere a source of interferable light not polarized is utilized, apolarizing plate should be disposed in front of the information patterngenerating device.

As apparent from the foregoing description, the present invention may besummed up as follows;

1. An element which uses an irregular ferroelectric crystal, such as GdM00 and KDP, each set of end faces of said crystal as oppose to eachother being cut such that they are normal to any one of aband c-axis andthat the thickness -d between both the end faces with respect topermeating light wavelength A and where P represents a positive integralmultiple, and in which a voltage sufficient to invert spontaneouspolarization of said crystal is applied in the direction of the whichweakens an intensity distribution of diffracted 7 gent lights in thepermeating direction. A plurality of 6 (N) such crystal elements arearranged such that the crystal axis within the principal planes of theparticular crystal element is orthogonal to the incident polarizalaserlight, and the Fourier transform hologram is pre-' pared by the use ofpassing light.

Next, as the second step, the polarity of only the elements at the offbit positions is inverted, and a hologram pattern is exposed to light ina manner to be superposed on the hologram which has been exposed tolight at the first step. v

To the double-exposure hologram preparation, the function of the phaseshifter is effective at either step in the form of being correlated withthe information pattern. Therefore, duringhologram exposure theintensity concentration may be remarkably avoided in comparison with theprior art. Considering reconstructed light from the double-exposurehologram since two phase-inverted light waves come to reconstructionimage positions corresponding to the off bit positions, the imagepositions become dark as the result of interference. Ultimately, thereconstructed image appears as the intensity of light in conformity withthe on" and off pattern of the input information.

The image of the information pattern generating de- 1 vice which hasphases randomly disturbed by the method thus associated with the inputinformation pattern, is formed on the hologram recording medium.Thereafter, a voltage exceeding the coercive electric field of theparticular irregular ferroelectric substance is applied to only thoseelements of the pattern generating device which correspond to the bitsto be turned "off, thereby changing the senses of spontaneouspolarization of the elements by 180. The device is again subjected tothe permeation of light. Thus, the double exposure is carried out intothe hologram recordin medium.

I claim: 1. A reversible phase-modulating element comprismg:

an irregular ferroelectric crystal plate having a set of mutuallyopposing end planes, upon which light is directed, normal to one of theaand b-axis, and the thickness between said end planes is prescribed,with respect to the difference An in the refractive 0 and having theirpolarization planes respectively parallel to said aand b-axis within thecrystal and to a positive integer p, by

means for applying a voltage, sufficient to invert the spontaneouspolarization of said irregular ferroelectric crystal plate by 180 tosaid irregular ferroelectric crystal plate and in parallel to thec-axis. 2. A reversible phase-modulating element according plate is madeof Gd; M00,);,. 7

3. A reversible phase-modulating element according to claim 1, furthercomprising; polarizing means which is arranged in optical sequencebefore one end plane of one set of said mutually opposing end planes ofsaid irregular ferroelectric crystal plate of said reversible phase- LIIindex between light beams having a wavelength A to claim 1, wherein saidirregular ferroelectric crystal modulating element, a polarization planeof said polarizing means being arranged in parallel to one of said aandb-axis of said end planes of said irregular ferroelectric crystal plate.4. A reversible phase-modulating element according to claim 3, whereinsaid irregular ferroelectric crystal plate is made of Gd, M000 5. Apattern generating device comprising: a plurality of reversiblephase-modulating elements each comprising an irregular ferroelectriccrystal plate in which a set of mutually opposing end planes, upon whichlight is directed, is normal to one of the a-, band c-axis, and thethickness between said end planes is prescribed, with respect to adifference An in the refractive index between light beams having awavelength A and having their polarization planes respectively parallelto said aand b-axis within the crystal and to a positive integer p,by()t)/(2'An)(P+ /),and

means for applying a voltage, sufficient to invert the spontaneouspolarization of said irregular ferroelectric crystal plate by l80, tosaid irregular ferroelectric crystal plate and in parallel to saidc-axis,

said irregular ferroelectric crystal plates, which constitute therespective elements, being arranged on a plane normal to an incidentlinearly polarized beam so as to form a matrix among them, one of saidaand b-axis on said mutually opposing end planes of said each crystalplate being arranged in parallel to a.polarization plane of saidincident linearly polarized beam.

6. A pattern generating device of a construction according to claim 5,wherein said irregular ferroelectric crystal plate is made of Gd, MO

7. A pattern generating device comprising:

a plurality of reversible phase-modulating elements each comprising:

an irregular ferroelectric crystal plate in which a set of mutuallyopposing end planes, upon which the light is directed, is normal to oneof the a-, b and c-axis, and the thickness between said end planes isprescribed, with respect to a difference An in the refractive indexbetween light beams having a wavelength A and having their polarizationplanes respectively parallel to said aand b axis within the crystal andto a positive integer p, by

( 2 'An P+% ).and

roelectric crystal plate and in parallel to said c-axis,

polarizing means which is arranged in front of and in opposition to oneend plane of one set of said mutually opposing end planes of saidirregular ferroelectric crystal plate of said reversible phasemodulatingelement, a polarization plane of said polarizing means being arranged inparallel to one of said aand b-axis of said end planes of said irregularferroelectric crystal plate, said irregular ferroelectric crystalplates, which constitute the respective elements, being arranged on aplane normal to incident light, one of said aand b-axis on one set ofsaid mutually opposing end planes of said each crystalplate beingarranged in parallel to a predetermined linear polarization plane. 8. Apattern generating device of a construction according to claim 7,wherein said irregular ferroelectric crystal plate is made of Gd, M00

9. A reversible phase-modulating element comprising:

an irregular ferroelectric crystal plate in which a set of mutuallyopposing end planes is normal to one polarizing means which is arrangedin optical sequence before the crystal plate and in opposition to oneend plane of one set of mutually opposing end planes of said irregularferroelectric crystal plate. a polarization plane of said polarizingmeans being arranged in parallel to one of said aand b-axis of said endplanes of said irregular ferroelectric crystal plate.

1. A reversible phase-modulating element comprising: an irregularferroelectric crystal plate having a set of mutually opposing endplanes, upon which light is directed, normal to one of the a- andb-axis, and the thickness between said end planes is prescribed, withrespect to the difference Delta n in the refractive index between lightbeams having a wavelength lambda and having their polarization planesrespectively parallel to said a- and b-axis within the crystal and to apositive integer p, by ( lambda )/(2 . Delta n ) ( P + 1/2 ), and meansfor applying a voltage, sufficient to invert the spontaneouspolarization of said irregular ferroelectric crystal plate by 180* , tosaid irregular ferroelectric crystal plate and in parallel to thec-axis.
 2. A reversible phase-modulating element according to claim 1,wherein said irregular ferroelectric crystal plate is made of Gd2 (MoO4)3.
 3. A reversible phase-modulating element according to claim 1,further comprising: polarizing means which is arranged in opticalsequence before one end plane of one set of said mutually opposing endplanes of said irregular ferroelectric crystal plate of said reversiblephase-modulating element, a polarization plane of said polarizing meansbeing arranged in parallel to one of said a- and b-axis of said endplanes of said irregular ferroelectric crystal plate.
 4. A reversiblephase-modulating element according to claim 3, wherein said irregularferroelectric crystal plate is made of Gd2 ( MoO4)3.
 5. A patterngenerating device comprising: a plurality of reversible phase-modulatingelements each comprising an irregular ferroelectric crystal plate inwhich a set of mutually opposing end planes, upon which light isdirected, is normal to one of the a-, b- and c-axis, and the thicknessbetween said end planes is prescribed, with respect to a differenceDelta n in the refractive index between light beams having a wavelengthlambda and having their polarization planes respectively parallel tosaid a- and b-axis within the crystal and to a positive integer p, by (lambda )/( 2 . Delta n ) ( P + 1/2 ), and means for applying a voltage,sufficient to invert the spontaneous polarization of said irregularferroelectric crystal plate by 180*, to said irregular ferroelectriccrystal plate and in parallel to said c-axis, said irregularferroelectric crystal plates, which constitute the respective elements,being arranged on a plane normal to an incident linearly polarized beamso as to form a matrix among them, one of said a- and b-axis on saidmutually opPosing end planes of said each crystal plate being arrangedin parallel to a polarization plane of said incident linearly polarizedbeam.
 6. A pattern generating device of a construction according toclaim 5, wherein said irregular ferroelectric crystal plate is made ofGd2 ( MoO4)3.
 7. A pattern generating device comprising: a plurality ofreversible phase-modulating elements each comprising: an irregularferroelectric crystal plate in which a set of mutually opposing endplanes, upon which the light is directed, is normal to one of the a-, b-and c-axis, and the thickness between said end planes is prescribed,with respect to a difference Delta n in the refractive index betweenlight beams having a wavelength lambda and having their polarizationplanes respectively parallel to said a- and b-axis within the crystaland to a positive integer p, by ( lambda )/( 2 . Delta n ) ( P + 1/2 ) ,and means for applying a voltage, sufficient to invert the spontaneouspolarization of said irregular ferroelectric crystal plate by 180*, tosaid irregular ferroelectric crystal plate and in parallel to saidc-axis, polarizing means which is arranged in front of and in oppositionto one end plane of one set of said mutually opposing end planes of saidirregular ferroelectric crystal plate of said reversiblephase-modulating element, a polarization plane of said polarizing meansbeing arranged in parallel to one of said a- and b-axis of said endplanes of said irregular ferroelectric crystal plate, said irregularferroelectric crystal plates, which constitute the respective elements,being arranged on a plane normal to incident light, one of said a- andb-axis on one set of said mutually opposing end planes of said eachcrystal plate being arranged in parallel to a predetermined linearpolarization plane.
 8. A pattern generating device of a constructionaccording to claim 7, wherein said irregular ferroelectric crystal plateis made of Gd2 ( MoO4)3.
 9. A reversible phase-modulating elementcomprising: an irregular ferroelectric crystal plate in which a set ofmutually opposing end planes is normal to one of the a-, b- and c-axis,means for applying a voltage, sufficient to invert the spontaneouspolarization of said irregular ferroelectric crystal plate by 180*, tosaid irregular ferroelectric crystal plate and in parallel to saidc-axis, and polarizing means which is arranged in optical sequencebefore the crystal plate and in opposition to one end plane of one setof mutually opposing end planes of said irregular ferroelectric crystalplate, a polarization plane of said polarizing means being arranged inparallel to one of said a- and b-axis of said end planes of saidirregular ferroelectric crystal plate.